"G" is not referencing a box anywhere on the W-2. The Code "G" is the code you enter in box 12 to identify the amount you will enter in the section to the right of the code you enter. Weather you enter it on 12a or 12d doesn't matter. So for example, you enter the code "G" in the first part of box 12a, and the amount in the 2nd part of 12a.
